In three-dimensional printing, patterns are generally repeated on a printing surface, where successive drops on top of another eventually produce a three-dimensional object. Three dimensional printing has been most successful, to this point, in creating plastic objects. Three-dimensional printing of metal objects has been limited in its usefulness due to the technical difficulties in working with liquid metal.
Various methods of producing a liquid metal drop for printing have been developed. A number of devices known in the art utilize mechanical force to propel liquid metal out of a nozzle. Mechanical force for producing a drop can be generated by various means. Some devices related to the present disclosure utilize piezoelectric actuators to generate mechanical force to generate a drop, such as U.S. Pat. No. 7,077,334. The '334 patent is directed to a drop-on-demand printer. The method described in the '334 patent exemplifies the use of a piezoelectric actuator to create pressure in the fluid-containing chamber of a drop-on-demand printing device. Another example of the use of a piezoelectric actuator in drop-on-demand printing is described in U.S. Pat. No. 4,828,886. Means of producing a drop other than piezoelectric have been described in the related art. Ultrasonic means of generating a drop. Examples of this method include U.S. Pat. Nos. 3,222,776 and 4,754,900, which induce vibrations at the nozzle through the use of ultrasound to produce a drop.
The related art discloses various methods by which devices have utilized electromagnetic coils to produce a force on liquid metal to eject liquid metal out of a nozzle. For example, U.S. Pat. No. 6,202,734 relates to a device for producing liquid metal drops utilizing magentohydrodynamics. The '734 patent also describes the use of electromagnetic force to produce drop-on-demand liquid metal. The patentable improvement over the related art described by the '734 patent generally relates to the use of alternating current and magnetohydrodynamics in liquid metal printing.
A number of related art devices utilize a magnetic coil adjacent to the liquid metal to induce a field to impose a force on the liquid. In these types of devices, the liquid carries a current flowing in a direction perpendicular to the surrounding magnetic field, thereby generating a force. This type of device is generally known as an electromagnetic (EM) pump. EM pump devices generally rely on alternating current (AC) in the magnetic coil to produce a force on liquid metal. Examples of AC EM pump devices include U.S. Pat. No. 4,842,170; which describes an electromagnetic pump applying an alternating current to an electromagnetic coil adjacent a nozzle. U.S. Pat. No. 3,807,903 describes an electromagnetic pump that relies on varying electrical current to control the liquid flow from a nozzle.
U.S. Pat. Nos. 8,267,669, 4,818,185, 4,398,589; 4,566,859, 3,515,898 and 4,324,266, 4,216,800 also relate to devices for electromagnetically pumping liquid metal. Generally, these devices utilize alternating current or travelling magnetic fields by physically moving permanent magnets to impart force on a liquid metal. These devices were patentable because they improved upon the prior art by eliminating the need for solid electrodes to produce a current in the metal flow. The patentable improvements over the prior art for the '669 and '185 patents generally relate to the ability of the devices to create a force in the liquid metal stream without electrodes that could corrode, or seals that could fail.
U.S. Pat. No. 5,377,961 relates to an improvement on an electromagnetic pump type device for producing drops of liquid metal. The '961 patent relates to a soldering device for depositing small amounts of solder on a printed circuit board. The '961 device pinches off drops by a mechanism that propels a drop forward and reverses force on the stream to separate the stream from the drop using an AC current applied to the liquid metal. The improvement of the '961 device relates to the reversal of force to produce a drop in a relatively short period of time. The method utilized by the '961 device reverses the direction of the electric current applied to the system, causing the force exerted on the solder stream to be substantially instantaneously reversed without the necessity of transferring electrical energy to vibratory, ultrasonic or the like.
The related art described above has several disadvantages. The '734 patent does not utilize direct current (DC) applied to a magnetic coil to produce a force in an annular direction leading to the liquid metal being forced radially toward the nozzle, thereby producing a liquid metal drop. The use of a DC pulse to produce a force simplifies the construction of a drop-on-demand printer. With regard to the relevant art described previously, where mechanical force is used to generate a drop, seals and moving parts are prone to wear and failure. For example, a piezoelectric actuator must be kept below its curie temperature to continue functioning. This requires it to be placed remotely behind insulation and act through rods or linkages. This complexity adds friction, risk of leakage, low performance and more expensive maintenance requirements.
Similarly, mechanical means of displacing a drop generally involve more moving parts, which can lead to greater wear on the device and greater expense. Ultrasonic methods of mechanically displacing a drop rely on the back and forth motion induced by ultrasonic radiation. Such methods have not been effective enough to produce an economically viable liquid metal jet printer in the marketplace.
With regard to electromagnetic force devices, the related art described herein generally utilizes alternating current to generate an outward flow from the nozzle and a reverse, inward flow to displace the drop from the liquid stream. With the use of alternating current applied to a magnetic coil, the current must be applied in one direction and then the magnetic field must be reversed, a stepwise process that requires significant time, in terms of drop-on-demand printing and more complex and expensive power electronics. While related devices have addressed this issue, none have been successful in limiting exposure of critical parts to corrosive liquid metal which subjects such devices to significant and expensive wear.